Biaxially oriented polypropylene films (BOPP) used for packaging, decorative, and label applications often perform multiple functions. In a lamination they can provide printability, transparent or matte appearance, or slip properties. They sometimes provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties, provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
However, in recent years, interest in “greener” packaging has been strongly developing. Packaging materials based on biologically derived polymers are increasing due to concerns with renewable resources, raw materials, and greenhouse gases. Bio-based polymers are believed—once fully scaled-up—to help reduce reliance on petroleum, reduce production of greenhouse gases, and can be biodegradable as well. Bio-based polymers such as polylactic acid (PLA)—which is currently derived from corn starch (but can be derived from other plant sugars) and thus, can be considered to be derived from a renewable resource—is one of the more popular and commercially available materials available for packaging film applications. Other bio-based polymers scaling up are PHAs—polyhydroxy alkanoates—which are also of commercial interest.
For such a bio-based polymer to be fit-for-use for many snack food packaging applications, it is desirable that the bio-based polymer film match as many of the attributes possible that BOPP is well-known for, such as heat sealability, printability, controlled COF, metallizability, barrier properties, etc. A disadvantage that PLA films have in comparison to PP films is due to the higher density of PLA vs. PP: about 1.24 for PLA vs. 0.905 for PP. This means that a BOPLA film of the same thickness as a BOPP film will have a much lower yield—a term commonly used in the industry denoting a unit area per unit weight of the film—than the BOPP film. For example, a 20 μm (80 G) thick BOPLA film would have a yield of about 28,000 in2/lb (39.8 m2/kg) vs. a 20 μm (80 G) BOPP films yield of about 38,300 in2/lb (54.5 m2/kg). The difference in yield is due to the density difference between PLA and PP resin. Thus, this significantly reduced yield for BOPLA for a given film thickness makes BOPLA films much more costly to use when replacing BOPP film applications. Moreover, the cost of PLA resin is higher than PP to begin with. At the time of this writing, PLA resin is about 35-50% higher in cost.
One way to make BOPLA films more cost-competitive relative to BOPP films is to reduce the density of the BOPLA film which then improves its yield. One way to reduce density is via cavitation of the BOPLA film. However, the choice of cavitating agent can make an effect on film cost and density as well. Some mineral-based cavitating agents such as CaCO3 or talc has a higher density (about 2.7-2.8 for calcium carbonate and 2.75 for talc) than that of PLA, so adding CaCO3 or other mineral cavitators to the PLA film can offset the density reduction obtained by said cavitation. Moreover, due to the typically large variation in particle sizes, the voids or cavities produced around the mineral cavitating agent are often very large, irregularly shaped, and can adversely affect the mechanical and/or tear resistance properties of the film.
In addition, opaque or white opaque films are often desirable for certain packaging applications for aesthetic reasons. Such films provide a different appearance to the inside of the package when opened by the consumer (white look). A high opacity is also usually desirable so as to provide hiding power over the product, printing, or other laminate films; light or UV protection; or brighter white appearance.
EP Patent 1 385 899 B1 describes a sequentially biaxially oriented film composed of at least one layer including an aliphatic hydroxycarboxylic acid (i.e. PLA) and 0.5-30 wt. % of a cyclic olefin copolymer (COC) having a Tg of 70-270° C. with the film having vacuole-like cavities and a density of less than 1.25 g/cm3. However, the use of COC adds cost to the film as this material is expensive to use.
U.S. patent application Ser. No. 12/444,420 describes an opaque simultaneously biaxially oriented PLA film composed of at least one layer including a polymer of hydroxycarboxylic acid (i.e. PLA) and 0.5-30 wt. % of a cyclic olefin copolymer (COC) having a Tg of 70-270° C. This application states that COC polymers are “the only known effective vacuole formers in biaxially oriented PLA films.” Moreover, the use of COC as a cavitating agent adds cost to the film as COC materials are expensive and costly to begin with.
U.S. patent application Ser. No. 12/483,072 describes a method to cavitate biaxially oriented PLA films using inorganic particles and low transverse orientation temperatures. The cavities and voids in using this approach can be very large and can have a negative effect on the film's mechanical strength and tensile properties.
U.S. patent application Ser. No. 13/313,567 describes the use of polystyrene as a cavitating agent for biaxially oriented PLA films which helps reduce the overall density of said film.
U.S. Pat. No. 7,531,585 B2 describes the composition and use of metal salt-phosphorous compounds for use as nucleating agents in PLA resin. These compounds have been found to be effective in improving crystallization speed and finer dispersion of crystal sizes. However, there is no contemplation of using these materials as cavitating agents to produce cavitated PLA films.